Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common knowledge in the field.
State-of-the-art silicon solar cell production is based on highly automated belt-type conveyor configurations. The as-cut p-type Si wafer is typically first subjected to chemical etching which removes saw damaged layers from both front and back surfaces of the wafer or edge damage from the laser cutting of silicon ribbons and then passes through the consecutive process steps including phosphorous diffusion to form a p/n junction; the deposition and firing of an antireflection Si3N4 coating; the creation of the front-side metal contact grid and Al back-side metal contact. Other modifications to optimize the process are also used in variants of these solar cell process steps. Finally, the individual solar cells are connected sequentially in strings, to achieve required voltage and current, and the strings are laminated into solar panels.
The PV industry, with crystalline silicon as a dominant segment, is rapidly expanding to meet growing renewable energy demands all over the world. The silicon wafer is a major contributor to the overall cost of the solar cell: currently up to 75% of the overall cost. One of the major technological problems is the identification and elimination of sources of wafers' mechanical defects such as thermo-elastic stresses and cracks leading to the loss of wafer integrity and ultimately to the breakage of as-grown and processed Si wafers and of PV cell based on these wafers.
The price of silicon raw material has grown substantially in the last three years due to a world-wide shortage of polycrystalline silicon feedstock. To compensate for the feedstock shortage, solar Si wafers are sliced thinner to a thickness of less than 100 microns [J. Wohlgemuth, M. Narayanan, R. Clark, T. Koval, S. Roncin, M. Bennett, D. Cunningham, D. Amin, J. Creager “Large-scale PV module manufacturing using ultra-thin polycrystalline silicon solar cells” Conference Record of the Thirty-First IEEE Photovoltaic Specialist Conference (IEEE Cat. No. 05CH37608), 2005, Pages 1023-1026]. Wafer areas have also been increased to reduce overall production costs, and larger sizes, up to 210 mm×210 mm, are now available.
Thinner and larger wafers are, however, more difficult to handle during production, this leads to a reduction in yield due to increased breakage especially in high speed automated manufacture. In-line wafer breakage reduces equipment throughput as a result of down time required for cleaning in-line equipment, and removing broken wafers from fixtures.
There is, therefore, a recognized need for devices and a methodology for fast in-line quality control methods and apparatus. Common problem leading to wafer breakage is related to small cracks that under thermal or mechanical stress cause wafer's mechanical fracture. Further, it is recognised that impact of a particular crack on wafer's mechanical property depends on the size of the crack and its location within the wafer.
The majority of the methods presented in the prior art are based on imaging techniques, comprising capturing and processing an image of a wafer in order to determine its spatial irregularities.
Scanning Acoustic Microscopy (SAM) is an imaging technique using 150 MHz pulses for precise identification and visualization of micro-cracks as small as 10 microns. The cracks are identified as acoustic impedance discontinuity of wafer at the crack region [M. C. Bhardwaj “Principles and methods of ultrasonic characterization of materials” Advanced ceramic materials, 1 (1986) pp 311-324]. The steps of SAM technique including, immersion of wafer into de-ionized water, mapping of the pulse amplitude, and data analyses are each relatively slow, so that the full testing procedure even in its automated version can occupy several minutes of precious manufacturing time. This evidently makes SAM unsuitable for in-line applications where no more than a few seconds per wafer is acceptable for quality inspection.
Another approach is offered by an optical inspection imaging where relatively large cracks are visualized by a light transmission technique [E. Rueland, A. Herguth, A. Trummer, S. Wansleben, P. Fath “Optical micro-crack detection in combination with stability testing for in-line-inspection of wafers and cells”, Proceedings of 20th EU PVSEC (Barcelona, 2005) pp. 3242-3245]. This technique, however, lacks the capability to observe small cracks at the wafer's periphery. The optical inspection is also non-applicable to processed wafers having back-side Al contact and to complete solar cells. An additional limitation of the transmission technique is that tightly closed cracks with width of ˜1 micron are not detectable due to the optical diffraction limit.
Recently reported data on luminescence imaging [T. Trupke, R. A. Bardos, M. C. Schubert, W. Warta, “Photoluminescence imaging of silicon wafers”, Applied Physics Letters (2006), Volume 89, Issue 4, 44107; T. Fuyuki, H. Kondo, T. Yamazaki, Yu. Takahashi, Yu. Uraoka “Photographic surveying of minority carrier diffusion length in polycrystalline silicon solar cells by electroluminescence” Appl. Phys. Letters 86, 262108 (2005)] is particularly developed for testing of indirect band-gap semiconductor devices such as silicon solar cells. As an imaging technique the method's speed is limited by implemented image recognition software, which needs to perform a substantial computational task of analysing complex luminescence image of a wafer. Another drawback of the method is that other defects such as surface scratches and dislocation slip lines can be misinterpreted as cracks thus leading to false positive answers. The application of this method to identification of electrically isolated or poorly connected regions such as those caused by breaks in the metal pattern was disclosed in details in an international patent application PCT/AU2007/000595.
Ultrasonic lock-in thermography is sufficiently sensitive, however it requires a longer measuring period for signal averaging due to low infrared intensity [J. P Rakotoniaina, O. Breitenstein, M. H. Al Rifai, D. Franke, A. Schnieder “Detection of cracks on silicon wafers and solar cells by lock-in ultrasound thermography”, Proceedings of PV Solar conference (Paris, June 2004), pp. 640-643].
A new non-imaging experimental algorithm for fast crack control using Resonance Ultrasonic Vibrations (RUV) was disclosed in the paper [A. Belyaev, O. Polupan, W. Dallas, S. Ostapenko, D. Hess, J. Wohlgemuth “Crack detection and analyses using resonance ultrasonic vibrations in full-size crystalline silicon wafers”, Appl. Phys. Letters 88, 111907-1 (2006). The RUV approach for stress control in silicon wafers was also disclosed in the U.S. Pat. No. 6,413,789 B2 [S. Ostapenko “Method of detection and monitoring stresses in a semiconductor wafer” U.S. Pat. No. 6,413,789 B2]. The method, as described in prior publications, allows for fast detection of wafer's imperfections but is not applicable for in-line control. The method involves measurement of a single resonant curve and correlating one parameter of the curve with the internal stresses or cracks in a wafer. The wafers, however, vary in a range of physical parameters such as lateral dimensions, thickness, and shapes. While small variations of these parameters are acceptable within the quality requirements for PV cells, these variations often lead to false positive events when an acceptable wafer is falsely recognised as a potentially breakable wafer with cracks. Further, the method is not capable to provide an information in relation to the location of crack. Above mentioned shortcoming limit benefits of using the method of U.S. Pat. No. 6,413,789 for in-line quality control of wafers.
Another method, based on impact testing is disclosed in the U.S. Pat. No. 5,257,544 titled “Resonant frequency method for bearing ball inspection”. This invention provides a method for detecting defects in test objects which includes generating expansion inducing energy focused upon the test object at a first location, thereby causing pressure wave within the test object. At a second location, the acoustic waves are detected and the resonant frequencies' quality factors are calculated and compared to predetermined quality factor data. The inventors claim that such comparison provides information of whether the test object contains a defect. Once again, the method operates with a single rejection parameter, which, when applied to wafers, limits its ability to distinguish cracked samples from statistically variable samples, leading, therefore to high proportion of false positive event unacceptable in manufacturing practice. Further the method requires high precision in locating an incoming acoustic pulse (impact) and of a sensor detecting acoustic waves. This precludes identification of cracks located in proximity to the selected positions. Crucially, an impact testing by a single or multiple acoustic pulses is less sensitive than techniques based on periodical sinusoidal excitation. The periodic excitation allows for substantial reduction of signal-to-noise ratio by synchronizing frequency and phase of a detected response to an excitation with that of a reference signal causing the excitation. Furthermore, the impact testing has high probability to creating new cracks in standard silicon wafers when focused ultrasonic beam hit the wafer close to areas of high internal stress.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.
Therefore, the present invention addresses the need for fast, accurate and non-destructive determination of mechanical defects in wafers, including detecting and locating cracks in wafers, particularly applicable as a diagnostic in-line tool in solar cell production.